Hundreds of African boys have disappeared from London schools, police investigating the murder of a boy whose torso was left in the Thames have said.

Scotland Yard asked London education authorities how many black boys aged four to seven had vanished from school.

Between July and September 2001, 300 had disappeared, and police fear thousands may go missing annually.

Child welfare experts say the figures hint at the scale of child trafficking, sometimes for labour or benefit fraud.

A previous BBC investigation found some African children were being held by their parents' creditors, so they could claim extra benefits.

'Lost in the system'

The boy found in the River Thames in September 2001 - called Adam by police - is thought to have been the victim of a ritual killing after being brought to London from Nigeria.

Detective Chief Inspector Will O'Reilly said inquiries revealed 300 black boys of a similar age to Adam had not reported back to school in the three months before his death and were "lost into the system".

They are being trafficked, they are being emotionally abused, there are incidences of domestic slavery, there is physical abuse, sexual abuse

"It is a large figure - far more than we anticipated when we started this line of inquiry," Mr O'Reilly said.

Of the missing boys, 299 came from Africa and one from the Caribbean.

Despite an international search, police were able to find only two of them, Mr O'Reilly said. Most of those questioned said the children had returned to Africa.

While there is nothing to suggest these children have been murdered, a lack of immigration records makes them almost impossible to trace, police say.

'Underground' problem

Chris Atkinson, policy adviser on child protection for the National Society for the Prevention of Cruelty to Children said children needed to be tracked upon arrival in the UK and called for a register of approved private foster carers.

We have quite a large number of parents or families who apparently go missing but actually just move on

The BBC's Angus Stickler says part of the problem lies with the custom in many African communities of placing children with distant relatives.

The government does not currently force adults who take children in these circumstances, known as private fostering, to register. It fears doing so could drive the problem underground.

Felicity Collier, of the British Association for Adoption and Fostering, said the problem was already underground and she called for tighter controls.

But Tim Benson, head teacher at Nelson Primary School in East Ham, London, and a National Association of Head Teachers representative, said most children were probably happily ensconced in another school.

"We have quite a large number of parents or families who apparently go missing but actually just move on and don't know that they have to inform anyone," he said.

Children's register

A spokeswoman for the Department for Education and Skills said she could not comment on details of the investigation but the government was "deeply concerned" about any child missing from education.

Under the 2004 Children's Act - being implemented this year - local authorities are meant to advise people to tell social services if they are caring for a child.

Police and welfare groups hope this will make it easier to track vulnerable children.

Shadow secretary of state for children Theresa May said the Children's Act needed to go further, with a system allowing for prosecution of those who failed to register as carers.